Conventionally, ophthalmic lenses have been used as optical elements for vision correction in the human optical system and as alternative optical elements after crystalline lens extraction. Among them, contact lenses applied to the eye and intraocular lenses to be inserted therein have been used extensively because they provide a wide vision by being directly used for the human eye while reducing the uncomfortable feeling in seeing objects.
Meanwhile, there are increasing number of people in recent years who reached the presbyopic age and continue to wear contact lenses. Since such seniors who suffer from presbyopia have their focal functions deteriorated, they develop a symptom of hardly being able to focus on objects nearby. Therefore, presbyopia patients will need multifocal contact lenses that allow them to focus on nearby objects, too. Also, since patients who underwent a cataract surgery have their crystalline lens removed that used to adjust the vision, they still have symptoms resulting in difficulties in seeing nearby objects even if intraocular lenses are inserted in their eyes. It is becoming necessary for such intraocular lenses to have a multifocal function realized by multiple focal points. Thus, needs for multifocal lenses are increasingly growing in recent years reflecting our aging society.
As methods of producing such multifocal ophthalmic lenses, there have been known a refraction-type multifocal lens that forms multiple focal points based on the refraction principle and a diffraction-type multifocal lens that forms multiple focal points based on the diffraction principle. In the latter mentioned diffraction-type multifocal lens, the optical part of the lens is provided with a plurality of diffraction structures formed concentrically, and multiple focal points are formed by the effect of mutual interference between light waves that pass through the multiple diffraction structures (zones). Therefore, such lenses have an advantage of being able to set a higher lens power while minimizing the lens thickness as compared to refraction-type lenses that generate focal points by the refraction effect of light waves at the refracting interface, which is a boundary of different refractive indices.
Generally speaking, the diffraction-type multifocal lens has a diffraction structure where the pitch of diffraction zones gradually gets smaller as it moves from the center toward the periphery according to certain rules called ‘Fresnel pitch,’ and the 0th order diffracted light and first-order diffracted light generated from the structure are used to produce multiple focal points. Usually, the 0th order diffracted light focal points for far vision while the first-order diffracted light focal points for near vision. By providing such a distribution of diffracted light, a bifocal lens having focal points for far and near vision can be produced.
However, the diffraction-type ophthalmic lens has a problem of easily generating band-like or ring-like circles of light around the light source when the light source is viewed by an eye from a distance at night. This circle of light usually called ‘halo’ tends to appear around a point light source such as a street light or a motor vehicle headlight or the like in a distance, which causes a problem of deteriorated visibility at night using the ophthalmic lens. The halo is one of the phenomena reflecting the imaging characteristics of multifocal lenses, especially those called the simultaneous perception-type, and the cause of the halo formation can be explained as follows:
In case of an ideal monofocal lens with no aberration, light from far distance passes through the lens and focuses an image at a given focal point position so as to intensify the amplitudes of light waves each other to the maximum extent (FIG. 43A). In that process, the image plane intensity distribution at the focal point position shows a simple pattern of only a main peak at the center with very small side lobes defined by the Airy radius existing around it (FIGS. 43B, 43C). FIG. 43C is a magnified view of FIG. 43B. Therefore, when a light source is viewed from a distance, an image is formed with no halo that reflects such intensity distribution (FIG. 43D).
Meanwhile, a diffraction-type multifocal lens having two focal points for far and near vision is designed in such a way that the light from a distance produces an image at the far focal point position so as to maximize the amplitude of light waves each other, while intensifying the amplitude of each other at the near focal point position, too. Light from a distance forms the main peak centered around the image plane at the far focal point, whereas light waves intensified each other at the near focal point position diverge thereafter to reach the image plane at the far focal point (FIG. 44A). At a first glance of FIG. 44B, there seems to be only one main peak on the image plane at the far focal point, but as shown in the magnified view of FIG. 44C, a group of small peaks can be recognized. As mentioned above, these peaks are formed by the light components focusing at the near focal point to be mixed in the far focal plane as a kind of stray light. Thus the intensity of the group of small peaks is very small compared to that of the main peak, but even light with the smallest intensity can be conspicuous in the night environment with dark background, and further, the image can be easily detected by the retina with high visual sensitivity to be perceived as a halo (FIG. 44D).
Such formation of a group of small peaks occurs as a phenomenon of light waves, and as shown in FIG. 45A, the light passing through each diffraction zone of a diffraction-type multifocal lens exhibits an amplitude distribution reflecting the characteristics of each zone on the far focal point image plane. For example, the light passing through each of the zones A, B and C in FIG. 45A forms the amplitude distribution shown in FIG. 45B. Then, a composite of amplitudes of the light beams from each zone determines the overall amplitude distribution on the far focal point image plane (FIG. 45C). The conjugate absolute values of these amplitudes become the intensity of light (FIG. 45D) to be perceived by the eye as the group of small peaks described above. Therefore, in order to reduce the intensity of the group of small peaks, it is necessary to restrict the amplitudes or the expanse thereof on the image plane in the underlying amplitude distribution, and controlling such amplitude distribution leads to the halo reduction.
Some other background arts propose a solution to the halo problem addressed with regard to the diffraction-type multifocal ophthalmic lens. Japanese Domestic Publication of International Patent Application No. JP-A-2000-511299 (Patent Document 1), for example, discloses a method of smoothly reducing the height of the diffraction zone toward the periphery in a diffraction structure composed of one form of diffraction zone called ‘echelette’ in order to reduce the halo as well as a function that defines such changes in height. This method tries to reduce the amount of energy distributed to the near focal point as it moves toward the periphery and reduce the halo as a result. However, in the background art mentioned above, the amount of energy distributed to the near focal point needs to be substantially lowered in order to reduce the halo to an imperceptible level, in which case there is a problem that the visibility of near objects is significantly deteriorated. Also, because of the changes in energy ratio between far and near focal points following the changes in pupil size, there is a problem of difficulties in providing a constant visibility under a condition of changing illuminance.
Also, Japanese Unexamined Patent Publication No. JP-A-2007-181726 (Patent Document 2) discloses a multifocal ophthalmic lens that blocks or reduces the transmission of blue light and/or near-UV light in order to eliminate glare and halos. In such background art, scattering of light is considered to be the cause of the glare and halos, which can presumably be reduced by preventing the transmission of short-wave light subject to scattering. However, since the halo is attributable more to the intrinsic behavior of light for forming a near focal point rather than the scattering of light, the background art does not produce a basic solution to the problem even though some ancillary effects can be expected. Thus, under the current situation, there is no such thing like a diffraction-type multifocal ophthalmic lens with restricted halo generation and a good balance between far and near vision.